1. Field of the Invention
The invention relates in general to an apparatus and a method for converting a digital signal into a corresponding analog signal, and more particularly to an apparatus and a method used in a liquid crystal display for executing gamma correction.
2. Description of the Related Art
Recently, liquid crystal displays (LCDs) have been widely used because they have favorable advantages of thinness, lightness, and low electromagnetic radiation.
The LCD monitor has a plurality of pixels arranged in an array. Each pixel is composed of an upper plate, a lower plate, and a liquid crystal layer between the upper plate and the lower plate. Liquid crystal molecules are filled between the upper plate and the lower plate to form the liquid crystal layer. The upper plate and the lower plate have electrodes. When voltages are applied to the electrodes of the upper plate and the lower plate to generate a voltage difference between the upper plate and the lower plate, the orientations of the liquid crystal molecules in the liquid crystal layer may vary with the change of the voltage difference. The orientations of the liquid crystal molecules may affect the ratio of light transmitting through the pixel, which is called light transmissivity. The magnitude of the light transmissivity determines the brightness of the pixel. As the light transmissivity increases, the pixel becomes brighter. Therefore, by controlling the voltage difference between the upper plate and the lower plate, different pixels on the LCD monitor may have different brightness.
Please refer to FIG. 1, which shows the gamma relation between the light transmissivity and the voltage difference between the upper plate and lower plate of the pixel. The relation between the light transmissivity and the voltage difference between the upper plate and lower plate is non-linear, as shown by the gamma curve of FIG. 1. The voltage difference between the upper plate and lower plate is called the gamma voltage. In addition, the light transmissivity only relates to the magnitude of the gamma voltage, but has nothing to do with the polarity of the gamma voltage. Hence, the gamma curve is composed of a positive-polarity gamma curve 102 and a negative-polarity gamma curve 104, both of which are symmetrical with respect to the longitudinal coordinate. If two gamma voltages with the same magnitude but different polarities are applied to a pixel, the pixel may have the same light transmissivity under the two conditions. If the gamma voltage with fixed polarity is applied to each pixel continuously, the liquid crystal molecules of the pixel may be damaged. Therefore, it is possible to protect the liquid crystal molecules by alternating the polarities of the gamma voltages applied to the pixels.
In general, the pixel data input to the LCD is binary digital data. Since the relation between the gamma voltage and the light transmissivity of the pixel is non-linear, the LCD needs a particular circuit device for converting the digital pixel data into corresponding driving voltage to the upper plate or the lower plate according to the gamma curve so that the relations between the values of the pixel data and the light transmissivity of the pixel are linear. The above-mentioned operation is called the gamma correction, which may enhance the display quality of the LCD monitor.
Please refer to FIG. 2, which is a schematic illustration showing the gamma correction principle. When the gamma correction is performed, multiple sets of pixel data are selected as reference pixel data. In FIG. 2, the pixel data D0, D1, D2, D3 and D4 serve as the reference pixel data. According to the gamma curve, each reference pixel data corresponds to a positive-polarity reference voltage and a negative-polarity reference voltage, respectively. Taking the reference pixel data D0 as an example, it corresponds to a positive-polarity reference voltage V0 and a negative-polarity reference voltage V9. Similarly, the five sets of reference pixel data D0 to D4 correspond to the five positive-polarity reference voltages V0 to V4 and the five negative-polarity reference voltages V9 to V5, respectively, as shown in FIG. 2. The general pixel data mentioned above, is 8-bit binary data and may be represented as 256 gray-scale values. During gamma correction, the corresponding relation between the reference pixel data and the reference voltage may be used as the basis to derive driving voltage corresponding to all other pixel data by way of an interpolation method. Each pixel data may correspond to a positive-polarity driving voltage and a negative-polarity driving voltage.
It should be noted that the driving voltage corresponding to each pixel data becomes more precise as the number of selected reference pixel data for gamma correction increases. In general, eight sets of reference pixel data are selected to perform gamma correction. According to the gamma curve, eight sets of reference pixel data correspond to eight positive-polarity reference voltages and eight negative-polarity reference voltages, respectively. The gamma correction apparatus may perform the gamma correction on the basis of these sixteen reference voltages.
Please refer to FIG. 3, which is a schematic illustration showing the conventional gamma correction apparatus 300. The gamma correction apparatus 300 includes a gamma correction circuit 302 and a reference voltage generating circuit 304 coupled to the gamma correction circuit 302. The reference voltage generating circuit 304 has a resistor string composed of 17 resistors r1 to r17 connected in series. The first and final nodes of the resistor string are coupled to the voltage source 304. Each node of the resistor string may output reference voltages Vr, including eight positive-polarity reference voltages Vr(+) and eight negative-polarity reference voltages Vr(−) by properly controlling the resistance values of the resistors. Each reference voltage Vr is output to the gamma correction circuit 302 through the buffer BUF. The gamma correction circuit 302 outputs corresponding driving voltage VGM by gamma-correcting each pixel data using an interpolation method based on the reference voltages Vr.
The conventional reference voltage generating circuit 304 outputs a set of reference voltages Vr for the gamma correction circuit 302 to perform gamma correction using resistors to divide the voltage. For a color LCD, pixels on the monitor represent red (R), green (G), and blue (B), respectively. The pixels representing different colors may not have the same gamma curves. In addition, for a large-scale LCD monitor, since the degree of manufacturing difficulty increases, the gap distance between the upper plate and lower plate in the entire LCD monitor are difficult to keep the same. The gap differences between the upper plate and the lower plate may adversely influence the gamma curves for the pixels. Therefore, the gamma curves of all pixels are not completely the same on a large-scale LCD monitor.
In sum, the gamma curves of the pixels on the LCD monitor may be different from one another because the pixel colors and the gap distances between the upper plate and lower plate are not completely the same. If only one fixed reference voltage is output according to only one fixed gamma curve to gamma-correct all pixel data, the whole LCD monitor may represent undesirable frame colors, which are not identical to the ideal frame colors. Such an occurrence of color shading may cause the LCD to have display quality that is not optimum.